In processing plants, gases and liquids are routed through various conduits and tanks. In order to control the operation of the plant, the current status of the liquids and gases in the various portions of the plant as well as the state of controlled devices such as valves and burners must be monitored. The variables monitored to determine these states are referred to generally as process variables and can include pressures, differential pressures, temperatures, tank levels, valve positions, and the like. These process variables are determined using one or more sensors which generate an electrical signal indicative of the process variable. Because processing plants are dynamic, the electrical signals produced for the process variables are also dynamic and tend to vary significantly over time.
In the past, filtering had been used to remove or dampen the variations in the process signals. More recently, statistical process monitoring (SPM) has been introduced in which the dynamics of the process variables are used as one measure of the state of the process variable. Under SPM, a median or mean of the process variable signal can be determined to represent the process variable itself while a standard deviation or coefficient of variation can be calculated to provide a measure of the dynamic nature of the process variable.
Some process variables have a normal amount of variation or a normal standard deviation. When the standard deviation increases above or significantly below the normal amount of standard deviation, it can be indicative of abnormal conditions within a portion of the processing plant. For example, if the standard deviation of a differential pressure sensor drops, it can be indicative of a plugged impulse line.
The statistical values, collectively referred to as SPM data, are generally determined by electronics within field devices such as sensors, transmitters and valve controls that are located within the processing environment. These field devices can be configured to generate alerts when the SPM data crosses a threshold indicating abnormal operation. These alerts are sent to a host system, which can make the alert visible to an operator. Alternatively, the field device can transmit the SPM data to the host system via a digital communication protocol. The host system can be configured to generate alarms when the SPM data crosses various thresholds. In addition, multi-variant statistical analysis can be performed on the SPM data at the host system.